During photographic copying, it is generally customary to measure the individual originals before the exposure with respect to their density and color distribution and to determine from the measuring results the required exposure for each of the three color emulsions of the copying material which results in a copy of the original to be copied that is true to the original colors. The process used for this purpose is based on the so-called principle of the integration to gray which assumes that the colors of a copy integrate to neutral gray over the whole image surface because the scene of the picture taken typically contains the same proportions of all three basic colors red, green and blue. Because of this principle, a constant equilibrium of color is maintained in the copies if the exposure aims at maintaining a constant relationship of the basic-color proportions in the copying light.
The measuring of an original to be copied is therefore limited to a color-selective measuring of the integral transmission density in the colors red, green and blue and their comparison with that of a reference original for which the exposure values are already determined in order to carry out corresponding corrections of the exposure values as a function of a miscellaneous deviation. The method of operation virtually has the aim of producing, for a number of originals to be copied, the same color equilibrium that the reference original has.
For the integral density measuring of originals to be copied, light measuring instruments, commonly called color analyzers, are available which are predominantly used in combination with photographic enlarging and copying apparatuses. In this case, for the homogenizing of the measuring light, a diffusing lens is introduced into the projection light shining through the original and the light intensity in the individual basic colors in the copying plane is measured by means of a color-selective measuring probe.
The integral measuring process has the advantage that it is simple because the measuring device which is required for this purpose is relatively simple and the carrying-out of the measurement, particularly the positioning of the measuring probe, is not critical. However, the integral analysis on the basis of the mean color density of the original is found to be unsatisfactory in a case in which the image to be reproduced has a strong dominant color. In such a case, the criterion of always restoring in the copy the color equilibrium of the reference original in reality results in a adulteration of the color of the copy in comparison to the original. At least in some of the originals to be copied, the integral measurement results in an unsatisfactory color reproduction which requires a repetition of the exposure with correspondingly corrected exposure values. The object of obtaining a yield of satisfactory first copies that is as high as possible by means of measuring the originals is therefore not achieved.
From the DE-A 3512744, a photometric rotary sensor for translucent originals is known which permits a point-by-point scanning of the original. On the curved shell surface of a rotary cylinder, which carries out a simultaneous rotating motion and linear motion, several light sensors are mounted to which the measuring light is guided by way of light guiding rods which reach up to the picture original. The electric signals of the sensors are received at the rotary cylinder by way of collector rings and are supplied to an electronic storage and analyzing circuit, in which case, by way of corresponding coding elements, every signal arriving in the storage device can be assigned to a fixed measuring point on the original. The point-by-point measuring of the whole picture surface permits a more precise detecting of the color distribution in the original than the integral measuring and, if suitable criteria are used for evaluating the signals, leads to an increased yield of satisfactory first copies. The described sensor device with the combined rotating and linear movement and the receiving of the sensor signals from a rotating sensor support prove relatively expensive and cumbersome, particularly in the case of larger-format originals if the entry surfaces of the light guiding rods are moved along the picture surface in an x-y-movement.
So-called video analyzers are also known which are equipped with a separate measuring light source and furnish a positive control picture of an original on a video screen. They permit a direct evaluation of the visualized picture and the adjustment of the picture color by means of appropriately influencing the signals. Thus a basic calibration is carried out again with respect to a reference original in order to obtain information concerning densitometric differences between the original to be copied and the reference original and to determine from that, in a computing device connected behind it, the adjustments to be carried out at the enlarging or copying apparatus. By means of such a video-analyzing device, good results can be achieved as a rule with respect to satisfactory first copies with respect to the yield. However, the complexity of the system and the connected high expenditures considerably limit the range of application.